1. Field of the Invention
The present invention relates to a multi-directional sound signal reproducing apparatus, and particularly to an apparatus for compensating for distortion or noise of a musical sound included in a disc which is engraved with a signal multiplied by a subchannel signal and a main signal.
2. Description of the Prior Art
There have been proposed several types of system as a multi-directional sound signal reproducing apparatus. In this connection, we have previously proposed the system using encoding signals in co-operation with Duan H. Cooper which system is publicly known. The above encoding signals will first be briefly described for easy understanding of the invention.
Now, a reference line O--O as shown in FIG. 1 is imaginatively set in a sound field and a sound source at a position having an angle .theta.' relative to the reference line O--O is taken as S(.theta.'). If this localization accuracy is expressed as a function of azimuth .theta.', it is developed into Fourier series as follows: EQU S(.theta.') = a.sub.0 + a.sub.1 cos .theta.' + a.sub.2 cos 2.theta.' + . . . + b.sub.1 sin .theta.' + b.sub.2 sin 2.theta.' + . . . (1)
If it is developed with the expression of exponential function using complex number, the following equation (2) is obtained: EQU S(.theta.') = a.sub.0 + c.sub.1 e.sup.J.theta.' + c.sub.2 e.sup.J2.theta.' + . . . + c.sub.-1 e.sup.-J.theta.' + c.sub.-2 e.sup.-J2.theta.' + . . . (2)
the above mentioned localization accuracy of S(.theta.') can be expressed more precisely by increasing the number of terms in the above equation (2). However, the aforesaid encoding signal of the equation (2) is considered as an electric signal expressed by the following equation with respect to a phase angle .theta. whose value is the same as that of the azimuth .theta.'. EQU S(.theta.) = a.sub.0 + c.sub.1 e.sup.J.theta. + c.sub.2 e.sup.J2.theta. + . . . + c.sub.-1 e.sup.-J.theta. + c.sub.-2 e.sup.-J2.theta. + . . . (3)
in the equation (3), for signal transmission it is enough to transmit the number of coefficients corresponding to the channel number, and for n-channel transmission it is enough to transmit coefficients of the equation (3) including nth term at maximum. With respect to a plurality of sound sources S.sub.1, S.sub.2, S.sub.3 . . . , these coefficients of respective terms are respectively expressed as follows: EQU a.sub.0 = T.sub..SIGMA. = S.sub.1 + S.sub.2 + S.sub.3 + . . . (4a) EQU c.sub.1 = T.sub..DELTA. = S.sub.1 e.sup.-J.theta..sbsp.1 + S.sub.2 e.sup.-J.theta..sbsp.2 + S.sub.3 e.sup.-J.theta..sbsp.3 + . . . (4b) EQU c.sub.-1 = T.sub.T = S.sub.1 e.sup.J.theta..sbsp.1 + S.sub.2 e.sup.J.theta..sbsp.2 + S.sub.3 e.sup.J.theta..sbsp.3 + . . . (4c) EQU c.sub.2 = T.sub.Q = S.sub.1 e.sup.-J2.theta..sbsp.1 + S.sub.2 e.sup.-J2.theta..sbsp.2 + S.sub.3 e.sup.-J2.theta..sbsp.3 + . . . (4d) EQU c.sub.-2 = T.sub.5 = S.sub.1 e.sup.J2.theta..sbsp.1 + S.sub.2 e.sup.J2.theta..sbsp.2 + S.sub.3 e.sup.J2.theta..sbsp.3 + . . . (4e)
By way of example, when 4-channel signals are transmitted, the signals expressed by coefficients a.sub.0 to c.sub.2 may be transmitted among those shown in the equations (4a), (4b) . . . The above-mentioned equation (4a) is called as T.sub..SIGMA. which is a sum signal independent of phase, and the equation (4b) is called as T.sub..DELTA. which corresponds to a difference signal of stereophonic signals.
Thus, in order to have compatibility with prior art stereophonic signals, the first- and second-channel signals are respectively taken as follows: EQU T.sub.R = T.sub..SIGMA. + T.sub.66, T.sub.L = T.sub..SIGMA. - T.sub..DELTA. ( 5)
if the equation (5) is substituted by the equations (4a) and (4b) and only one sound source S.sub.1 is taken into consideration, the equation (5) is rewritten as follows: ##EQU1## If a condition S.sub.1 = 1 is assumed, the following equations are obtained: ##EQU2## When these signals T.sub.R and T.sub.L are transmitted and 4-channel reproduction thereof is carried out at the reproduction side, it is called as BMX reproduction. In this case, the 4-channel reproduced sound field is stable, but the separation is poor.
Further, when the signal T.sub.T of the equation (4c) is transmitted together with the signals T.sub.R and T.sub.L and 4-channel reproduction thereof is carried out at the reproduction side, it is called as TMX reproduction. In this case, the separation is improved as compared with that of the BMX reproduction.
Furthermore, when the signal T.sub.Q of the equation (4d) is transmitted together with the signals T.sub.R, T.sub.L and T.sub.T and 4-channel reproduction thereof is carried out at the reproduction side, it is called as QMX reproduction. In this case, we have ascertained that the separation is further improved as compared with the TMX reproduction.